Event Location:

The ongoing Indo-Asian collision poses an enormous seismic hazard to growing populations in the region. The prevailing tectonic model for the central Himalaya holds that ~2 cm/yr of Indo-Asian plate convergence is accommodated along the primary decollement under the range, the Main Himalayan Thrust (MHT). During the interseismic period, the MHT is locked from the surface to ~100 km down-dip. Accumulated strain is released during rupture of this locked zone, which can generate destructive Mw > 8.0 earthquakes. Over longer timescales, transport of the orogenic wedge over a mid-crustal ramp in the MHT drives rapid rock uplift and exhumation rates in the Greater Himalaya. This tectonic model, developed primarily from studies in central Nepal, is often projected along strike with little regard for lateral variations in MHT geometry or associated rock uplift patterns. In this talk I will present multiple lines of evidence for a major discontinuity in the MHT in west Nepal. Analysis of topographic metrics and microseismicity suggests that the band of rapid rock uplift above the MHT ramp in central Nepal bifurcates in west Nepal, resulting in a two-step topographic front. We cautiously interpret this bifurcation as the expression of uplift over two ramps bounding a young, active duplex on the MHT. Our interpretation is supported by the presence of a high-elevation, low-relief, relict landscape between the two ramps. Erosion-rate data from cosmogenic radionuclides confirm that this slowly-eroding, low-relief zone is experiencing a transient wave of incision as it adjusts to its new relative base level. Preliminary U-Th/He cooling-age data support rapid exhumation above the northern topographic step, but not the southern. However, it is possible that uplift over the southern ramp is so recent that young cooling ages recording this acceleration have not yet reached the surface. In any case, this heretofore unrecognized segmentation of the MHT in west Nepal has important implications for regional seismic hazard and for models of orogen evolution.